1. Field of the Invention
The present invention relates to a laser device, in particular, to a current injection type laser device for a frequency band ranging from a millimeter wave band to a terahertz wave band (30 GHz to 30 THz).
2. Description of the Related Art
As a new type semiconductor laser, a semiconductor laser called a quantum cascade laser which is based on a transition between energy levels (intersubband transition) of carriers in a certain energy band of a conduction band or a valence band has been known so far. An oscillation wavelength of the quantum cascade laser depends on an energy gap between two energy levels of optical transition. Thus, the oscillation wavelength can be selected from over a wide spectral range (for example, from a mid-infrared region to a terahertz band). At first, it was demonstrated that such a laser could be achieved by using a structure having a selected oscillation wavelength of 4.2 [μm] in the mid-infrared region. Recently, because there is a demand for an electromagnetic wave source of a terahertz wave band which is believed to be useful to bio-sensing and the like, there has been a development of a long wavelength laser in which the oscillation wavelength is shifted toward a longer wavelength from a mid-infrared region.
A long wavelength laser should have a structure which is provided with a gain medium for producing a gain within the frequency range, and achieve an optical confinement to the gain medium; for, a gain medium may have a layer thickness on the order of 1 [μm] to 10 [μm] at most, and a gain medium for a typical long wavelength laser has a thickness which is 1/10 or less of that of an oscillation wavelength. An approach for optical confinement by conductive cladding is known in the field of conventional semiconductor laser, but the use of this approach does not enable an optical confinement to a gain medium having a thickness which is smaller than the diffraction limit of light. Therefore, other types of optical confinement approaches have been proposed for oscillating a long wavelength laser.
Japanese Patent Application Laid-Open No. 2000-138420 discloses a method for solving the above problem. The cladding used in this method is a negative dielectric constant medium in which the real part of the dielectric constant is negative. In this method, the waveguide mode guided by the cladding is an electromagnetic wave due to the polarization oscillation of charge carriers (which is called a surface plasmon) in the negative dielectric constant medium. Since there is no diffraction limit in the surface plasmon, most of the modes can be confined to the gain medium which has a thickness on the order of 1/10 of an oscillation wavelength. By using such an approach, the method achieves a laser oscillation having an oscillation wavelength of 11.4 [μm], which is shifted toward a longer wavelength.
An example of the prior art is found in Benjamin S. Williams et al.; Appl. Phys. Lett., Vol. 83 (2005), 2124. This document discloses a method for arranging negative dielectric constant media in which the real part of the dielectric constant is negative, to the top and the bottom of a gain medium as a cladding. In this method as well, the waveguide mode guided by the claddings is a surface plasmon. The gain medium having two negative dielectric constant media as a cladding allows more modes to be confined to the thin gain medium which has a thickness on the order of 1/10 of an oscillation wavelength, compared to the case in Japanese Patent Application Laid-Open No. 2000-138420. By using such an approach, the method achieves a laser oscillation having an oscillation wavelength of about 100 [μm] (3 [THz]), which is shifted further toward a longer wavelength.
However, it is known that, in the conventional approaches for optical confinement, a confinement of a surface plasmon to a gain medium which is much thinner than an oscillation wavelength causes a relatively large waveguide loss. The waveguide loss increases a threshold of laser oscillation, which in turn causes other problems such as an increase of power consumption accompanied by an increased threshold current density, and a necessity of a separate cooling unit for maintaining the laser oscillation. Because the thinner gain medium increases the threshold of laser oscillation, a gain medium had to have a relatively large thickness for the conventional long wavelength laser. That is, a gain medium which is used in a quantum cascade laser and is formed with a semiconductor multi-layer film including hundreds to thousands of layers is required, but such a gain medium is relatively expensive, and has increased the cost of the conventional long wavelength laser.
The above problems are particularly acute in the case of a long wavelength laser of 100 [μm] or more (3 [THz] or less) which requires an optical confinement to a gain medium having a thickness of 1/10 or less of an oscillation wavelength.